Combining over the frequency range is a pain, so the upper power limit
is that of a single device in most cases.
Impedance, impedance, impedance. The electrical problems are actually
pretty nasty with parasitic inductance starting to dominate as the frequency
rises.
Higher impedance MOSFET devices are emerging (1000V VDS), but they are
frequency
limited to about 100MHZ, and not capable of generating more than 150W or so. It
might
be possible to combine these devices as the impedances are high enough, but I
have not
tried this.
Even in single device designs at those frequencies, construction details and an
intimate
understanding of the sources of parasitic inductance are essential. "Cut and
try" methods
often result in a brief, pretty blue flash inside the FET and a slagged die or
bond wire ( I have a small
bag of slagged FETS). The drain-source varactor capacitance can also make life
interesting especially
at higher frequencies or in switch mode amplifiers.
You can reasonably do a KW at 50Mhz from a set of conventional 50V FETS. The
active parts will
cost around $500.00 and the power supply will cost more than that. Add
reasonable protection
circuitry and what not and the costs will equal those of the high end
commercial 6m amplifiers.
Funny thing is... I like semiconductor amplifiers and have two under
construction at the moment for
amateur application and have designed and built a couple for plasma generation
professionally.
But for lots of power at those frequencies, the high output impedance and
forgiving nature of a tube
makes it hard for amateurs to justify working with semiconductors.
A breakthrough will occur when the MOSFET amplifier can be supplied directly
from a rectified and filtered
line voltage (with a buck converter perhaps) and is capable of dissipating a
couple of hundred watts.
Of course, sucking that heat out from such a small area and flushing it will
still be difficult.
George K. Watson
K0IW
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